Steam heat and coil is used for new living area

When i bought my house the main part of the house is steam heat, but the extention that was added is baseboard heat where they used the coil to heat, had no problems till we built a small doorma above that room so the problem now is = they taped into the existing curc pump and what happens is if the that zone runs on its own it works fine, but when the steam side runs too the pressure builds up to about 40lb then the Relief valve leaks water please help